1. Field of the Invention
The present invention relates to a position sensor such as magnetic scales and rotary encoders, to be applied to machine tools, industrial machines, precision length/angle measuring instruments, etc. and a bias magnetic field generating device to be used in the position sensor.
2. Description of the Related Art
Hitherto, as a detection head of a position sensor such as magnetic scales and rotary encoders to be applied to machine tools, industrial machines, precision length/angle measuring instruments, etc., a magnetoresistance effect device (MR device) which utilizes the magnetoresistance effect of a thin film of Fe—Ni, Ni—Co or the like has been used.
The scales and rotary encoders using the MR device-based detection heads vary widely in shape and structure according to the purpose, use or the like thereof. Among the scales and rotary encoders, those which need high accuracy and high resolution are fundamentally so configured that the pitch (recording wavelength) of magnetic recording is shortened or the output waveform of the MR sensor is controlled to have a reduced harmonic distortion and an enhanced signal-to-noise ratio (S/N) so as to reduce errors at the time of interpolation within one wavelength, thereby realizing high accuracy and high resolution.
In addition, in such a detection head as above-mentioned, it is known that still higher accuracy and higher resolution can be realized by providing the MR sensor with a bias magnet so as to generate a bias magnetic field.
Meanwhile, the above-mentioned type of position sensor has a magnetic recording medium with magnetic information recorded thereon at a position opposed to the MR sensor, and position detection is performed by detecting the magnetic information on the magnetic recording medium by the MR sensor when the MR sensor and the magnetic recording medium are moved relative to each other. As the magnetic recording medium to be used in this type of position sensor, one that includes an incremental layer and an absolute layer has been proposed. The magnetic recording medium including the incremental layer and the absolute layer is generally of a two-track configuration in which one incremental layer and one absolute layer are provided in parallel to each other. Further, for the purpose of reducing the phase difference between the absolute layer and the incremental layer, there has been proposed a magnetic recording medium of a three-track configuration in which two incremental layers and one absolute layer interposed therebetween are arranged in parallel to each other.
Here, in the case where the position sensor including the detection head for detecting the magnetic information recorded on the magnetic recording medium having a plurality of tracks is provided with the above-mentioned bias magnetic field generating section, the MR sensors corresponding respectively to the absolute layer and the incremental layer(s) can be supplied only with the same bias magnetic field. Therefore, if the bias magnetic field is designed according to the incremental layer, the sensitivity of the MR sensor in relation to the absolute layer would be lowered and the accuracy in detecting the magnetic information would be worsened. On the other hand, if the bias magnetic field is designed according to the absolute layer, interpolation accuracy and return error with regard to the incremental layer are worsened considerably.
[Patent Document 1]
Japanese Patent Laid-open No. Hei 8-68661